Satellite systems are playing an increasing role in mobile communications by providing coverage in zones where land-based infrastructures are unable or ineffective to supply mobile services. Satellite systems can, for example, provide coverage in large, sparsely-populated areas where it is not cost effective to implement network infrastructures. Satellite systems can also make services available to airborne and sea-based users. Another use of satellite systems is to back-up or supplement a land-based network. Satellite systems can continue to provide services to users when segments of the network infrastructure are down, can carry additional traffic during peak transmission periods when the land-based network is overloaded, and can fill-in holes in the coverage area of a land-based system caused by man-made or natural obstacles.
To meet their potential, satellite communication systems must provide reliable connections and must not experience frequent interruptions. As with conventional land-based mobile communication systems, satellite systems require a great deal of signalling overhead to establish, maintain, and terminate connections for all the services offered. The signalling protocols used in satellite systems are typically based on GSM, a derivative of Signalling System No. 7 (SSN7), which is adapted for the mobile environment. SSN7 is a protocol for fixed network signalling and exchange switching established by CCITT (International Telegraph and Telephone Consultative Committitee). For use in GSM and other mobile communication systems, SSN7 was adapted and extended, for example, by adding procedures to cope with the mobility of the user. The GSM technical specifications are published by the European Telecommunications Standard Institute (ETSI) and are incorporated in their entirety by reference herein.
The signalling protocol in GSM includes a large number of carefully defined signalling procedures. A signalling procedure consists of a series of signalling messages that are exchanged between various entities. The structure and elements of each signalling message are defined by the protocol. In the case of call control messages (which are used in the connection management sub-layer), the signalling message includes a transaction identifier which serves as a label for the transaction so that multiple transactions can be managed.
At the beginning of a transaction, a free TI is assigned to the transaction by the side of the interface initiating the transaction. The assigned TI value is used for the duration of the transaction and is released upon termination of the transaction. A mobile unit may support up to eight transactions at a time, distinguishing signalling messages belonging to different transactions by their TI values.
In satellite communication systems there is a problem with synchronizing transaction identifiers used by two mobile terminals during a single-hop connection. A single-hop connection in a satellite communication system is a connection between two mobile units. To establish a single-hop) connection, a dual-hop connection must first be established. The originating unit first connects to the network via the satellite which then connects to the terminating mobile unit via the satellite. Each of the call setup connections, originating mobile-to-network and network-to-terminating mobile, is assigned a TI value. At the end of the call setup signalling, the network commands both mobile units to connect to a single radio channel to establish the single-hop connection between the mobile units. There is no guarantee that the TI value used by each mobile station to identify the transaction will match each other. If the TI values do not match, then the mobile units will not be able to recognize signalling messages from the other mobile unit during the single-hop connection. The mobile units could be instructed to ignore the TI values during a single-hop connection. However, if the TI value is ignored, then only one transaction could be handled at a time. This would severely restrict the services that could be provided over the network.
There are several possible solutions to the problem described above. Since the same mobile switching center (MSC) manages the initial connections to the network by each of the mobile units, it would be possible for the MSC to choose a transaction identifier which is not being used by either mobile unit and command the mobile units to use that transaction identifier. This approach, however, requires new MSC functionality. Another solution would be for the mobile unit to agree upon a new transaction identifier after the mobile unit has connected to the radio channel. This solution, however, would require new signalling messages and would prolong the call setup time.